1. Field of the Invention
The present invention relates to electrophotographic-type image forming apparatuses.
2. Description of the Related Art
Electrophotographic-type image forming apparatuses form static images upon the surface of a photosensitive member by irradiating the photosensitive member with a laser beam (a light beam) emitted from a laser light source. At that time, the laser light source is driven based on an on/off signal (a pulse width modulated signal; called a “PWM signal” hereinafter) in accordance with image data, and therefore the laser light source is either in a turned-on state (an on state) or a turned-off state (an off state). Generally speaking, the current driving scheme and the voltage driving scheme exist as methods for driving a laser light source. The current driving scheme is a driving scheme that controls a current so that the current applied to the laser light source is constant. Although the current driving scheme is advantageous in that relationships between driving currents and emitted light intensities can be determined uniquely, and control is therefore easy, the light emitting response characteristics of the laser light source drop as the resistance value of an internal resistor provided in the laser light source increases. On the other hand, the voltage driving scheme controls a voltage so that the voltage applied to the laser light source is constant. Although the voltage driving scheme exhibits superior light emitting response characteristics, a voltage source is necessary for controlling the light amounts of the individual laser beams of a surface emitting laser, which is likely to lead to an increase in the scale of the circuit. Thus far, close-to-ideal driving control has been realized by exploiting the merits of both the voltage driving scheme and the current driving scheme and switching between the driving schemes based on the on/off signal. An invention in which the voltage driving scheme is used during the rise time or fall time of an on signal in a PWM signal and the current driving scheme is used in periods following the rise time or fall time of the on signal has been proposed (Japanese Patent Laid-Open No. 2008-098657).
It is desirable for the light emitting response characteristics of a laser light source, or in other words, the amount of time required for the light amount of the laser beam to rise to a predetermined value (that is, the rise time), to always be constant relative to the on/off signal that drives the laser light source. The reason is that if the light emitting response characteristics are not constant, the shapes of the dots will also not be constant. However, realistically speaking, the temperature conditions of a laser light source differ depending on the amount of time the laser beam is turned on, the amount of time the laser beam is turned off, the emitted light intensity of the laser, and so on, and as a result, the light emitting response characteristics of the laser light source (that is, the rise time) are not constant. For example, the longer the laser light source is turned off, the more the light emitting response characteristics of the laser light source will drop when the laser light source is turned on thereafter. With a method in which a control system, which controls a laser light source that emits a laser beam, monitors the terminal voltage of the laser light source and corrects the driving amount of the laser light source, the light emitting response characteristics depend on the response characteristics of the control system. In other words, a slow response speed in the control system will lead to a longer rise time for the light beam. In the case where the repeat cycle for turning the laser light source on/off in accordance with the PWM signal is extremely short, such as several tens of ns, the conditions required of the control system in terms of response speed become fairly unrealistic.